Modern materials present a number of formidable challenges to the fabricators of a wide range of optical, semiconductor, and electronic components, many of which require precision shaping, smoothing, and polishing. Current methods, such as conventional grinding and polishing, have a number of disadvantages. Physical contact methods, such as grinding, abrasive polishing, diamond turning and ion milling, involve physical force at the microscopic scale and can create damage in the subsurface of the material being treated. Physical contact methods also have a trade-off between speed and quality. Smooth surfaces can require the use of very slow material removal rates, while hard materials such as silicon carbide can be extremely difficult to polish. Soft or delicate structures can also be difficult to polish, as the physical force involved can crack or bend the structures. Some materials such as glass can also end up with a surface layer of redeposited material, which can affect the properties and behavior of the manufactured component.
Damage-free Laser Optics
In one example of such a manufacturing challenge, optics produced with current or prior art polishing methods cannot withstand the high intensity of light produced by high-powered laser systems. One of the engineering challenges in such an advanced system is the need for a large number of defect-free optics to be produced within an acceptable period of time and at an acceptable cost. Subsurface defects in such an optic can cause cracks to form on the rear surface of lenses exposed to high ultraviolet laser light levels. These cracks can grow until a large fraction of the light is obscured or until the lens fractures. Some of these lenses also serve as a vacuum barrier, making catastrophic failure a serious safety concern.
Conventional abrasives-based polishing can be used for many materials. This polishing process is both chemical and mechanical, involving surface and solution chemistry as well as mechanical abrasion. Mechanical abrasion rapidly removes material, but can produce sub-surface damage and cause the damage to propagate deeper into the workpiece. The chemical portion dissolves and redeposits glass, forming a relatively smooth surface. The chemical kinetics of redeposition favor the formation of smooth surfaces, as high spots are mechanically abraded away while holes are filled through redeposition.
This process of redeposition can lead to problems in some applications. Analysis of the redeposition layer reveals a tremendous number of contaminants, mostly from the abrasive but also from previous polishing steps. This redeposition layer can affect the adhesion and physical properties of optical coatings. Below this redeposition zone can be an underlying zone of damaged glass, up to tens of microns thick or more. When high fluxes of light pass through this zone, damage sites can nucleate and grow, eventually leading to failure of the entire optic. The quality of the polish, and the underlying redeposition layer and subsurface damage, ultimately control how much light can be transmitted through the optics.
In order to produce optics capable of routinely withstanding laser intensities as high as 12 J/cm2, a process is required to remove the 20-30 microns of damaged material. Conventional polishing can be used to remove this damage layer, but it must be done very slowly—on the order of about 0.1 μm per hour. Polishing for this length of time also necessitates periodic checks of the shape of the part using precision metrology.
Wet Etching
Another approach to removing the damage layer in an optic is a wet chemical etch. In such a process, only a limited amount of material can be removed before the surface becomes excessively pitted, with a resulting increase in the amount of light scattered by the optic. Optics processed by wet etch have been tested, with the disappointing result that the damage threshold was unaffected.
Ion Milling
Another approach utilizes ion milling after conventional polishing. Ion milling is a well-established technique for removing small amounts of material from a surface using a kinetic beam of ions. Some advantages of ion milling include: no surface contact, no weight on the optic, no edge effects, and correction of long spatial wavelength errors.
There are numerous disadvantages to ion milling, however, including high surface temperatures, an increase in surface roughness, and the need for vacuum. The temperature is dependant on beam current, so that an increase in etch rate produces an increase in temperature often surpasses several hundred ° C. Nearly all heat must be removed through the chuck, usually requiring a good thermal connection between the workpiece and the holder. This is difficult when working on transmission optics because they must be held by the edges so as not to damage the polished surface. Further, ion beams cannot smooth surfaces. For small amounts of material removal, roughness can be held constant. Large amounts of material removal cause an unfortunate increase in roughness.
Reduced Pressure Plasma Methods
Another approach involves plasma etching at reduced temperature, which is used extensively in the semiconductor industry for processing of a wide variety of materials including semiconductors, metals and glasses. Reactive ions are believed to be responsible for the majority of material removal, leading the technique to be known as reactive ion etch (RIE). Considerable effort has been put into developing plasmas with uniform etch rates over the entire discharge, making RIE unsuitable for the production of figured precision components. The greatest practical drawback to RIE for precision finishing of optical components is the need for vacuum and a low material removal rate. Translating either the source or workpiece with precision on a complicated path inside a vacuum chamber is challenging, especially in the case of large optics. In-situ metrology would also be awkward.
A modified RIE for polishing at reduced pressure has been built using a capacitively coupled discharge. Named “Plasma Assisted Chemical Machining” (PACE), the system has been successful in shaping and polishing fused silica. While the parts polished by PACE have shown no evidence of subsurface damage or surface contamination, it has been found that greater sub-surface damage present before etching resulted in an increased roughness after etching.
A major limitation of this capacitively-coupled discharge approach is the requirement that the workpiece be either conductive or less than 10 mm thick. In addition, etch rates are dependant on part thickness, decreasing by a factor of ten when thickness changed from 2 to 10 mm. Above 10 mm the rates are too low to be of much use. If metrology is needed in an iterative procedure, the chamber must be vented and pumped down for the next etch step. The convergence rate for PACE is also typically very low, resulting in a long, expensive multi-step process. PACE technology was recently improved by the substitution of a microwave plasma source for the capacitively coupled system, but the rates are still too slow for optics manufacturing.
Atmospheric Pressure Plasma Methods
In yet another approach, a direct current (DC) plasma can be used at atmospheric pressure to thin wafers. Originally called a “Plasma Jet” and also referred to as Atmospheric Downstream Plasma (ADP), such a system uses argon as the plasma gas, with a trace amount of fluorine or chlorine for reactive atom production. The main intent of the device is to do backside thinning of processed silicon wafers for smart card and other consumer applications. With the ADP tool, wafers are thinned in a batch mode by placing them on a platten and using planetary type motion to move the sub-aperture plasma in a pseudo-random fashion across the surface.
Unfortunately, atmospheric DC plasma jets such as ADP are not well suited for the precise shaping and smoothing of surfaces or for material deposition. Because the reactive gas is mixed with the bulk gas prior to excitation, the reactive species in the plasma are widely distributed across the discharge. This substantially increases the footprint and the minimum feature size that can be etched into a surface. Furthermore, the electrodes that are used to establish the arc are eroded by the reactants. This adds particulates to the gas stream, as well as causing fluctuations in plasma conditions, and accounting for the reduced uniformity compared to RIE systems. Detrimental electrode reactions also preclude the use of oxygen and many other plasma gases.
Another plasma process, known as Chemical Vapor Machining (CVM), is a radio frequency (RF) plasma process that has been used to slice silicon. This plasma is generated around a wire or blade electrode immersed in a noble gas atmosphere containing a trace of reactive components. Like the PACE process it closely resembles, material removal through CVM is entirely chemical in nature. The damage for CVM and wet chemical etching are similar, close to the intrinsic damage typically found in silicon used in the semiconductor industry.
Several performance characteristics limit the applications of CVM. First, the non-rotationally symmetric nature of the footprint makes the process difficult to model and control. Process rates are limited by the rate at which the plasma converts the reactive precursor gas into radical atoms. The device is difficult to scale up, limiting the maximum removal rate and the practical limit for fine-scale material removal. While no vacuum is required for CVM, the workpiece must be enclosed in a vessel to contain the plasma atmosphere.
Another type of plasma jet has been developed to etch and deposit material on surfaces as well as to clean surfaces, known as an “ApJet.” This system consists of two concentric electrodes that generate a DC plasma which exits through a nozzle. The discharge is at a low temperature, making the process suitable for cleaning temperature-sensitive materials. The ApJet is not suitable for precisely shaping and polishing surfaces, as etch rates are low and the electrodes and nozzle erode and deposit material onto the surface. This makes precision control difficult. Furthermore, the ApJet cannot smooth rough surfaces.